Water in the Ahnet Basin (Algeria): Traces of the Wet Past of the Sahara Desert as an Analog for the Hesperian-Amazonian Transition in Arabia Terra (Mars)

Introduction 

During the whole Holocene, the Sahara Desert underwent a series of cyclic climatic fluctuations resulting in alternating arid and humid phases. Current arid conditions began ~6,000 BP. Prior to that, from ~14,500 to ~6,000 BP, widespread and abundant rainfall characterized the so-called ‘Green Sahara’, the last African Humid Period (AHP) when the Sahara was vegetated. [1, 2]. Evidence of the extensive pluvial activity that characterized the AHP can be documented even in areas close to presently hyperarid zones such as the Tanezrouft. The Ahnet Basin, situated at the northeastern margin of the Tanezrouft, is indeed characterized by a wide range of morphologies and deposits that testimony the wet past of the basin. 

Our work focused on establishing the relationship between the paleohydrography of the basin, still visible, and its Quaternary deposits to infer, through mapping and stratigraphicrelationships:

• A better understanding of how the wet to arid climatic transition influenced the deposits and their distribution inside the basin;

• The possibilities to use the Ahnet Basin as a terrestrial analog for the evaporitic, alluvial, and eolian deposits directly related to the wet-to-arid transition on Mars at the end of the Hesperian.

 

Geological and Geographical Setting 

The Ahnet Basin (~65,000 km²) lies in the Algerian Sahara, ~1,200 km south of Algiers, [3, 4] and is one of the ‘Peri-Hoggar Basins’ that formed during the Pan-African Orogeny (600–500 Ma). The metamorphic basement of Precambrian age is unconformably covered by Paleozoic successions of siliciclastic sediments up to 7 km in thickness. They consist of NNW-dipping sandstones and shales with few Devonian and Carboniferous limestone intercalations. The origin of the deposits is predominantly marine with continental fluviatile facies gradually transitioning to shallow marine and then to offshore moving from South to North [5, 6, 7]. The Mesozoic sedimentary record, due to a strong Triassic erosional phase, is completely absent from the basin. Finally, the Quaternary deposits, which we intend to analyze, appear to reflect a range of different genetic agents.

 

Data and Methods 

The study was performed inside a GIS environment and, focusing onfour areas originally selected to be part of a sampling campaign. We produced four hybrid geomorphological maps, one for each area, making use of true color Sentinel-2A images as basemap (resolution: 10 m/px) and the Copernicus 30 m DEM for topographic and morphological characterization. The hydrography was digitalized both manually and automatically using GRASS GIS. Stratigraphic relationships were studied using Google Earth imagery and GIS 3D models.

 

Results 

Mapping at 1:10,000 scale identified ~10 litho-morphological units across all study areas (see Fig. 1 for example). Tectonic structures were also mapped in this phase, while a more detailed characterization of the geomorphological elements will be presented in a future publication. This process revealed the extensive presence inside the basin of alluvial deposits directly linked to a dense and well-developed valley network. Despite the limitations set by our DSM in the analysis of such a pervasive hydrography, we managed to automatically extract channels up to the 11^th Strahler order. These data, however, clash with the present-time climate whose numerical parameters were collected from three weather stations located all across the basin. Precipitation is <15 mm/yr (Fig. 2), confirming hyperaridity. Current valley reactivation is likely only during rare catastrophic events. Instead, in good accordance with these numbers, is the existence of evaporitic and eolian deposits which can also consistently be found in all areas.

Stratigraphic relationships revealed periodic channel reactivation and desiccation involving alluvial and evaporitic deposits. Fig. 3 depicts, for example, the interaction between an alluvial fan and a sabkha inside one of the study areas. The numerous instances of reactivated channelized activity, rather than to some episodic precipitation events, could possibly point to a slow climatic transition from humid to arid. Further investigations is needed to confirm the climatic interpretation, this association of deposits appears to be, from both a morphological and stratigraphical point of view, an analog for Arabia Terra’s light-toned layer deposits and inverted channels. As such, the study of the effect of climatic shifting on these terrestrial deposits could shed some insights on the Noachian-Hesperian transition on Mars.

 

Acknowledgements

We acknowledge financial support under the National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 1.1, Call (2.2.2022) by the MUR, funded by the European Union – NextGenerationEU– Project Title Tanezrouft salt flat deposits (Sahara Desert): a priority target for a Mars Sample Return mission – CUP D53D23002600006 - Grant Assignment Decree No. 962 adopted on 30/06/2023 by the Italian Ministry of Ministry of University and Research (MUR).

This study was carried out within the Space It Up project funded by the ASI and the MUR (Contract n. 2024-5-E.0 - CUP n. I53D24000060005).

 

References

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[3] Perron, P. et al (2018). Solid Earth, 9: 1239-1275.

[4] Mostefai R. et al. (2023). Iraqi Geological Journal, 56: 1-13.

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[7] Zieliński, M. (2012). Marine and Petroleum Geology, 38: 166-176.